Isolated on its own in a petri dish, L1 has acrobatic talent, automatically assembling itself into what scientists call a "viruslike particle" that looks just like HPV, but has "none of the bad stuff inside," says Dr. Eliav Barr, head of the HPV-vaccine program at Merck.

The company's vaccine approach in both cases is to use viruslike particles (VLPs) containing surface proteins that make the VLPs resemble a virus, thereby eliciting the proper immune response-even though the VLPs lack the genes needed to replicate themselves.